1. Field of the Invention
The present invention relates to a thin film transistor for use in an active matrix liquid crystal display panel, an input/output device as of a contact-type image sensor, a portable electronic instrument, etc., and also to a method for manufacturing such thin film transistor.
2. Description of the Related Art
In forming a thin film transistor (hereinafter also called TFT) on a substrate of glass, the hydrogenated amorphous silicon semiconductor TFT technology and the polysilicon TFT technology are currently available as typical technologies. According to the former, the maximum temperature of the fabrication process is about 300.degree. C., and a carrier mobility of approximately 1 cm.sup.2 /Vsec is obtained. According to the latter, using a high-temperature process analogous to the LSI process of about 1000.degree. C. using a substrate as of quartz, a carrier mobility of 30 to 100 cm.sup.2 /Vsec can easily be obtained. In case of the high carrier mobility, when the thin film transistor is applied to, for example, a liquid crystal display, it is possible to simultaneously form on the same glass substrate a pixel TFT driving each pixel and even a peripheral driving circuit section as well. The resulting thin film transistor is inexpensive and small-sized.
However, in the polysilicon TFT technology, inexpensive low-melting-point glass, which is originally suitable to the former process, cannot be employed in the above-mentioned high-temperature process. For decreasing the temperature of the polysilicon TFT process, the laser crystallization technology, the low temperature film (gate insulating film) formation technology and the low temperature interface (of insulating film and silicon) formation technology are sought after. Studies and developments have been made to solve the foregoing problems. To this end, IEEE ELECTRON DEVICE LETTERS, Vol. 15, No. 2, page 69, "High Performance Polycrystalline Silicon Thin Film Transistor Fabricated Using Remote Plasma CVD of SiO.sub.2,"(M. Sekiya, et al.) discloses a hybrid fabricating apparatus and method composed of two different fabrication processes: the laser crystallization for forming a good interface between an insulating film and a silicon layer at low temperature, and the remote plasma CVD for forming a silicon dioxide film. This hybrid concept aims to maintain a good insulating-film-and-silicon interface at low temperature by forming a gate insulating film on a high-quality polysilicon film, which is formed by the excimer laser crystallization without exposure to atmosphere, and by keeping the interface well clean. In this technological paper, as a polysilicon TFT fabrication process, the following process flow was proposed. After a source-and-drain layer (20 nm thick) has been formed in an island shape, a silicon layer (20 nm thick) is formed. Then, laser crystallization, hydrogen treatment (which comes immediately after laser crystallization) and fabrication of a first insulating film (SiO.sub.2, 100 nm thick) take place in succession. As a result, a good insulating-film-and-silicon interface is obtained at low temperature. After that, the silicon layer is patterned in an island shape. In addition, a second insulating film (100 nm thick) is formed to secure silicon-electrode isolation.
In the meantime, as reduction of the power output and the driving voltage of a liquid display, an image sensor or the like has been practical, it is increasingly becoming necessary not only to improve the performance of TFT by cleaning the interface but also to reduce the operating threshold voltage of TFT. And it has turned out from recent studies that thinning the gate insulating film is effective for reduction of the operating threshold voltage; such thinning method applied to a planar thin film transistor will now be described with reference to FIGS. 9(a) to 9(c) of the accompanying drawings. FIG. 9(a) is a top plan view of the planar thin film transistor, FIG. 9(b) is a cross-sectional view taken alone line A-A' of FIG. 9(a), and FIG. 9(c) is a cross-sectional view taken along line B-B' of FIG. 9(a). A first insulating film 5 patterned in an island shape is located on and formed continuously with a semiconductor layer, which is adapted to be formed into a source-and-drain region 3, 4 and a channel region 2, and then a second insulating film 6 and a gate electrode (hereinafter also called simply the gate) 7 are located over the first insulating film 2 so as to cover the semiconductor layer and the island of the first insulating film 5. Now for reducing the threshold voltage as mentioned above, it becomes necessary to reduce the total thickness of the two-layer gate insulating film 5, 6. This is because reduction of the two layers increases the capacitance so that an adequate electric field effect can be obtained even at low voltage. Whereas the semiconductor layer, unlike the gate insulating film, cannot be thinned, partly because it should be doped with an impurity as by ion implantation and partly because an adequate process margin during laser crystallization should be secured. Consequently, if the gate insulating film is thinned in order to lower the threshold voltage as mentioned above, then the thickness of the second gate insulating film becomes smaller than the difference between the semiconductor layer and the first gate insulating film so that gate-leak-free covering is difficult to achieve. Thus short circuit (gate leak) would tend to occur between the gate and the source-and-drain region.
As a solution for these problems, the following method was proposed by Japanese Patent Laid-Open Publication No. Hei6-85258. As shown in FIG. 10 of the accompanying drawings of the present specification, a semiconductor film 12 is formed in an island shape on an insulator substrate 11 and then a first insulating film 13 is formed on the island-shape semiconductor film 12, whereupon a second insulating film 21 is formed so as to cover the stepped peripheral edges of such composite island of the semiconductor film 12 and first insulating film 13. And a gate electrode 14 is formed over the first insulating film 13 surrounded by the second insulating film 21, so the semiconductor film 12 and the gate 14 are perfectly isolated from each other, preventing occurrence of short circuit (gate leak) between the semiconductor film 12 and the gate 14, i.e., between the gate and the source-and-drain. To realize such structure, the second insulating film 21 has to be etched selectively, namely, at only the region directly above the island after having been formed along its entire surface over the first insulating film 13. As a consequence, the second insulating film 21 requires a material such that it can be selectively etched with respect to the first insulating film 13 or the second insulating film 21 requires a selective etching process. Especially when the first insulating film 13 is thinned further in an effort to improve the throughput and performance, a much higher selective ratio is needed, and a dry etching method free of plasma damage to either the gate insulating film or the insulating-film-and-semiconductor interface would be necessitated.
As an alternative method that ensures inter-electrode isolation using a two-layer insulating film, Japanese Patent Laid-Open Publication No. Hei6-61490 discloses a conventional two-layer technology which optimizes the thickness and cross-sectional shape of each layer and employs two highly dielectric thin films for securing high performance of a thin film transistor. As shown in FIG. 11, this conventional method aims to provide a well-insulating, highly reliable isolation structure by optimizing the shape of a second insulating film 1014b rather than the shape of a first insulating film 1014a, which covers gates 103a, 1013b. Further, the gate metal as a prospective lower electrode is covered with tungsten oxide and then a silicon nitride film for forming a good MIS interface with hydrogenated amorphous silicon, whereupon a hydrogenated amorphous silicon film and a source-and-drain layer are formed. This conventional covering method using the two-layer insulating film is effective in covering the stepped regions defined by only the gate electrode and semiconductor layer. However, this publication is totally silent about and even does not anticipate any second insulating film that covers the stepped edges of the island constituted by the semiconductor layer and the first gate insulating film.
As the low temperature formation technology for a gate insulating film in particular, developments of means for forming a silicon dioxide film by plasma CVD, sputtering or low pressure CVD have increasingly become popular. The silicon dioxide film, which is formed at such a low temperature of less than about 600.degree. C. as to enable the use of the above-described glass substrate, would be encountered with the following problems as compared to the thermally oxidized film to be used in the conventional LSI process.
For the bulk performance of the gate insulating film designated by (a) in FIG. 18 of the accompanying drawings of the present specification, it is required to reduce the defect level derived from residual stress, dangling bond, impurity or other cause and to improve the insulating strength.
For the high performance of interface with the silicon active layer constituting the channel designated by (b) in FIG. 18, it is required to reduce the interface level derived from incomplete cleaning, plasma damage, etc.
For covering the island's stepped edges designated by (c) in FIG. 18, the island-shaped semiconductor layer has to be covered precisely.
In order to solve these problems, a method of minimizing plasma damage with improving the insulating strength by sputtering using a mixed gas of oxygen and argon as a discharge gas has been proposed by Japanese Laid-Open Publications Nos. Hei3-120871 and Hei3-241873. However, in sputtering, since the film-forming precursor strikes on the substrate at about right angles, precise covering over the stepped edges of the island is difficult to achieve.
Japanese Patent Laid-Open Publication No. Hei3-19340 discloses a method for increasing the film-forming rate gradually as the film formation progresses away from the semiconductor-and-insulating-film interface, by lowering the concentration of helium with time during plasma CVD.
Japanese Patent Laid-Open Publication No. Hei3-108319 disclosed a method for forming a good semiconductor-and-insulating-film interface by carrying out the formation of a semiconductor thin film and the formation of an insulating film in a common reaction vessel.
The above-mentioned scientific paper, i.e., IEEE ELECTRON DEVICE LETTERS, Vol. 15, No. 2, page 69, "High Performance Polycrystalline Silicon Thin Film Transistor Fabricated Using Remote Plasma CVD of SiO.sub.2," by M. Sekiya, et al. Additionally discloses remote plasma CVD of silicon dioxide film formation for obtaining a good insulating-film-and-silicon interface. For forming a good semiconductor-and-insulating-film interface, plasma impact on the surface of a semiconductor, which constitutes a gate insulating film, is restricted by bringing a plasma forming region,-which serves to assist film-forming reaction of the precursor and dissolution of gas, away from the substrate position.
In this conventional method, however, although a good interface can be formed, the insulating strength is yet low so that thinning of the gate insulating film in an effort to secure the necessary reliability of the device and to lower the threshold voltage would be difficult to achieve.